There are two kinds of nematic liquid crystals; negative dielectric anisotropy nematic material and positive dielectric anisotropy nematic material. The former class of materials undergoes a dynamic scattering mode electro-optic phenomena above certain threshold voltage. In this class of material, the dielectric constant component parallel to the unique axis is smaller than the perpendicular component, thus its dielectric anisotropy is negative. In the second class of nematic liquid crystals, the dielectric constant component parallel to the unique axis is substantially larger than the perpendicular component, hence its dielectric anisotropy is positive. This latter class of materials does not undergo the dynamic scattering mode, instead will undergo dielectric realignment above a certain threshold electric or magnetic field. Thus the materials are field sensitive and its electro-optic phenomena is referred to as "Field Effect" phenomena.
The chemistry and certain of the physical and structural properties of liquid crystals have been studied (References 1-7).
Early liquid crystal chemists synthesized p-ethoxybenzylidene-p'-aminobenzonitrile. Castellano et al (Reference 10) continued the synthetic works on these compounds:
P-N-BUTOXYBENZYLIDENE-P'-AMINOBENZONITRILE,
P-N-HEXOXYBENZYLIDENE-P'-AMINOBENZONITRILE
P-N-HEXANOYLOXYBENZYLIDENE-P'-AMINOBENZONITRILE, AND
P-N-OCTANOYLOXYBENZYLIDENE-P'-AMINOBENZONITRILE.
All of these compounds exist as liquid crystals only at elevated temperatures and exhibit positive dielectric anistropy. Castellano et al found that some ternary mixtures among those compounds gave mixed liquid crystal which is stable at room temperature for extended periods of time. Some pleochroic dyes dissolved in those mixtures exhibited field tunable optical color filters.
Helfrich published a new electro-optic effect, commonly referred to as twisted nematics. He utilized a positive dielectric anisotropy nematic material, and as an example, he utilized a ternary mixture of Castellano's material. The most significant point of his experiment was that the display device can be operated at relatively lower voltages, e.g., 2-7 volts AC or DC. The electro-optic effect is based on the dielectric realignment, thus no dopants are necessary and high purity liquid crystal can be utilized, which is desirable and which will ensure the longevity of the operational life of the display. In addition, electric current flow is minimal (2-3 orders of magnitude lower than those commonly found in dynamic scattering mode), which minimizes any adverse electro-chemical reaction at the liquid crystal-electrode interface. He took advantage of another unique physical characteristic of nematic liquid crystal; due to the crystalline properties of liquid crystal, the molecules tend to associate into a directional pattern on a given substrate. This phenomena has been called "alignment". It was earlier found that if a pair of clear substrates, such as a microscope slide glass, are "rubbed" with dry cotton swab unidirectionally, and a few drops of liquid crystal is enclosed between thus prepared glass plates with their rubbed surfaces in direct contact with liquid crystal, the resultant thin film of liquid crystal exhibited so called "homogeneous" alignment. Under this condition, the rod-shaped liquid crystal molecules laid down on the surface with their major molecular axes parallel to the rubbing direction. The liquid crystal molecules in the bulk also followed parallel to those at the surface by their lateral attraction. The thus obtained liquid crystal medium behaves like a giant single crystal with its unique crystal axis parallel to the rubbing direction, and many physical and optical properties parallel and perpendicular to this unique axis were different.
Now, if the two glass plates are at right angle (90.degree.) to each other so that the rubbed direction at the inner surface of the top plate and the rubbed direction of the bottom plate makes right angle; and if a nematic liquid crystal is introduced in the thus prepared cell, a unique optical medium is obtained.
(Since this original work, many new techniques have been developed in obtaining the "Rubbing Effects", which are more amenable to manufacturing processes. Other cellulosics, synthetic or natural products, can be used instead of cotton swab. Permanently etched "micro-grooves", either by fine powder of abrasive materials, like diamond dust, or by photoetching, of the substrate give the same rubbing effects. More recently, some inorganic materials have been vacuum deposited at an oblique angle on the substrate, and have yielded excellent homogeneous alignment.)
The detailed electro-optical effects of the above "Twisted" optical medium, occurred in the following manner. As usual, the molecules at the immediate surfaces of the top and bottom plates will lie down with their major molecular axes parallel to the respective rubbing directions; however, since these directions are at 90.degree., with respect to each other the liquid crystal molecules in the bulk will tend, through their lateral interactions, to be conformed to the given environment. Thus the major molecular axis of the liquid crystal molecules will assume a helicoidal configuration between the top and bottom plates.
One of the most important physical properties of the above described optical medium is its ability to rotate plane polarized light by 90.degree. as the polarized light traverses through the medium. Of course, if the imposed angle is other than 90.degree., the angle of rotation of the plane polarized light will vary accordingly and angles other than 90.degree. are also found to be useful for display device fabrications.
If a linear polarizer is placed under the bottom plate, holding the direction of the polarization parallel to the rubbing direction of the bottom plate, and a flux of white light hits the bottom polarizer and is linearly polarized, it enters the liquid crystal medium without attenuation. But as the light wave front traverses the "twisted" liquid crystal medium, it follows the "directors" of the helically arranged nematic molecules and the emerging light is polarized perpendicularly with respect to the direction of polarization of the entering light. If a second polarizer is placed on top of the top plate of the cell, with its polarizing direction parallel to that of the polarizer placed at the bottom of the cell, the emerging light will be completely "extinguished". (Cells of this type have been described, see References 8-14.)
Electro-optic cells can be fabricated using a pair of glass plates with inner surfaces coated with transparent conductive film and filling the cell with a positive dielectric anisotropy nematic liquid crystal after proper surface treatment. In its quiescent state ("OFF" state), without electric power, the display appears dark in color. If an electric power of few volts (AC or DC) is applied across the two plates, the cell will appear transparent; in other words, the liquid crystal medium has lost its polarizing effect completely, thus the linearly polarized light from the bottom polarizer travels straight through the liquid crystal medium and the top polarizer without attenuation. The liquid crystal molecules undergo the dielectric realignment under the influence of the applied electric field, all the dipoles being aligned along with the field direction, which effectively destroys the quiescent helicoidal molecular arrangements. The liquid crystal layer now is called "uniaxial negative" and its optic axis (unique axis) is perpendicular to the cell surfaces. If the imposed electric potential is removed, the liquid crystal relaxes back to its helicoidal arrangement and its polarizing ability is attained quickly. Thus, the liquid crystal medium behaves like an electric controlled "light valve".
It is possible to fabricate various kinds of display devices, transmissive and reflective types. In the former case, the active segments appear clear on black background if the two polaroids are parallel and black segments on white background can be obtained with perpendicularly positioned polaroids. Ambient light source is enough to visualize, but back-lighting may be added to the display device to enhance the viewability. Reflective type displays, such as minimum power draining wrist watch displays, can utilize diffusive reflectors at the bottom of the display. The active segments will appear similar to those of transmissive mode displays, depending upon the relative position of the two polaroids. (See references 8-14 for descriptions of various liquid crystal devices of the type in which the materials of this invention are useful.)
There are several positive dielectric anisotropy nematic materials potentially useful for display devices. Castellano, Reference 13, described a homologous series of p-alkoxy and p-acyloxy-benzylidene-p'-amino-benzonitriles. These materials were used only for the electric field induced color filters but twisted nematics for field effect display devices have been described using positive dielectric anisotropy nematic materials as shown below: ##STR1## The binary mixtures A+B and C+D yield mixed nematic liquid crystals which are suitable for field effect display elements. Threshold voltage of 0.9V was obtained in the case of mixture A+B. The mixture A+B has another advantage; that is, this mixture is resistant to the chemical degradation due to moisture and is colorless, while the mixture C+D, and any other Schiff base materials, are very slightly yellow and sensitive to moisture contamination. More recently, another class of non-Schiff Base nematic materials, biphenyl derivatives, and mixtures of them were found to be useful as field effect display element.
In spite of the number of materials described, there are some more desirable material characteristics to be improved. These desirable properties include:
1. The nematic liquid crystal or mixtures should have wide nematic temperature range, including room temperature, preferably with room temperature in the middle of the range.
2. The nematic crystal or mixtures should have positive dielectric anisotropy of substantial magnitude, such that the resultant twisted nematic display device can be operated at commonly available driving voltages.
3. The electro-optic effect should start to appear at a definite voltage (Vth, threshold voltage) and reach its maximum at a slightly higher voltage (Vsat, saturation voltage) then the threshold value. Depending upon material system, Vth and Vsat change with different magnitudes by temperature changes of the surrounding environments, but these changes of Vth and Vsat should be minimal.
4. V - Vsat - Vth values for different materials indicate the sensitivity with which the materials respond to the applied voltage change. In order to obtain clear-cut electro-optic effect, which maximizes the multiplex capability of the display device, V should be minimum.
5. The materials should be chemically resistant to oxygen, moisture, ultraviolet light or electric current.
In addition, it has now been discovered that potentially serious shortcomings are inherent in the use of Schiff base materials as nematic liquid crystals. Such materials have very desirable electro-optic characteristics; however, they tend to be unstable under some conditions. Extreme care to avoid oxygen and moisture are required to obtain long term high stability liquid crystal components, and even the best practical care may not ensure a long life component. Schiff bases tend to decompose irreversibly and lose their electro-optic characteristics and, consequently, are not satisfactory materials in some liquid crystal devices. It is, therefore, desirable to provide liquid crystal materials which are less sensitive to degradation and which can be restored to a good nematic liquid crystal condition.